A linear polarization antenna includes a vertical polarization antenna that erects an element vertically to the ground to radiate or receive a vertical polarization wave and a horizontal polarization antenna that unfolds the element to the ground in a horizontal direction to radiate or receive a horizontal polarization wave.
Under an ideal environment without a reflection plate, and the like, the horizontal polarization wave and the vertical polarization wave transmitted by a transmitting module interfere less with each other, but cross-polarization discrimination (XPD) representing information on interference between both polarization waves has a value other than 0 under an actual environment.
The XPD represents, when a signal transmitted as the same polarization wave as a receiving antenna and a signal transmitted as a counter polarization wave are both received by the receiving antenna, a ratio between both signals and that the value is not 0 means that both types of polarization waves interfere with each other during transmitting both polarization waves.
When multiple subcarrier signals are transmitted through a multiple polarization antenna in order to prevent the interference between both polarization waves, subcarriers of the signals transmitted as the respective polarization waves may be configured to be shifted as long as a predetermined part of a subcarrier interval. When the subcarriers of the horizontal polarization signal and the vertical polarization signal are located to deviate from each other in a frequency domain, even in the case where the interference between both polarization signals occurs, a subcarrier of an interference signal is not fully superimposed with an original signal, and as a result, it is easier to remove the interference signal.
However, the shift of the subcarrier is helpful to reduce interference between different polarization waves at the time of transmitting data, but there is a higher possibility that training sequences transmitted by the respective polarization waves will give interference when the training sequences exclusively transmitted by the respective polarization waves are transmitted.
In the multiple subcarrier signals using the polarization wave, when a transmitting unit transmits the training sequence to one subcarrier only in one polarization wave and thereafter, a receiving unit receives the training sequence by each polarization antenna, a channel transfer function and an XPD value between the respective polarization waves can be known, and when the training sequences are transmitted with being superimposed with each other, it is difficult to accurately measure the interference between the polarization waves.